Airborne remote sensing assessment of the damage to cotton caused by spray drift from aerially applied glyphosate through spray deposition measurements

To identify the effects of aerially applied glyphosate on cotton plants, a field was planted with replicated blocks of cotton. To quantify relative deposits of applied chemical, spray samplers were placed in the spray swath and in several downwind orientations. An Air Tractor 402B spray aircraft, equipped with fifty-four CP-09 nozzles, was flown down the centre of the field to apply a mixture of 1.54 kg ha−1 of glyphosate and rubidium chloride tracer at a 46.77 l ha−1 application rate. At one week intervals following treatment, aerial colour-infrared imagery was obtained from the field using a global positioning system-triggered multispectral camera system. The processed drift and image data were highly correlated with correlation coefficients (r) from −0.38 to −0.97 at 1, 2, and 3 weeks after treatment. The drift and image data were used as the indicators of visual injury in regressions with a strong ability to explain variability (R2 from 0.36 to 0.90 for drift data for the first week after treatment (WAT); from 0.20 to 0.90 for image data from 1, 2, to 3 WAT). The results illustrate that spray drift sampling can explain early cotton injury (1 WAT), and airborne remote sensing can explain late cotton injury (2 and 3 WAT). The results are helpful for determining the extent of required near-field drift sampling and demonstrate that airborne multispectral imaging can be a viable tool for determining the extent of damage relative to derived concentrations of glyphosate.